It is important for polydienes which are used in many applications to have high vinyl contents. For example, 3,4-polyisoprene can be used in tire tread compounds to improve tire performance characteristics, such as traction. Polar modifiers are commonly used in the preparation of synthetic polydiene rubbers which are prepared utilizing lithium catalyst systems in order to increase their vinyl content. Ethers and tertiary amines which act as Lewis bases are commonly used as modifiers. For instance, U.S. Pat. No. 4,022,959 indicates that diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, and N-phenyl morpholine can be used as modifiers. U.S. Pat. No. 4,696,986 describes the use of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenes as modifiers. The vinyl group content of polydienes prepared utilizing Lewis bases as modifiers depends upon the type and amount of Lewis base employed as well as the polymerization temperature utilized. For example, if a higher polymerization temperature is employed, a polymer with a lower vinyl group content is obtained (see A.W. Langer; A. Chem. Soc. Div. Polymer Chem. Reprints; Vol. 7 (1), 132 [1966]). For this reason it is difficult to synthesize polymers having high vinyl contents at high polymerization temperatures utilizing typical Lewis base modifiers.
Higher temperatures generally promote a faster rate of polymerization. Accordingly, it is desirable to utilize moderately high temperatures in commercial polymerizations in order to maximize throughputs. However, it has traditionally been difficult to prepare polymers having high vinyl contents at temperatures which are high enough to attain maximum polymerization rates while utilizing conventional Lewis bases as modifiers.